Focal length range. Sharpness, contrast and saturation is comparable to the 24-70L. 3rd gen IS is excellent. Lightweight and weathersealing.

Cons:

Distortion, esp from 24-35. Vignetting (light fallout) in the corners on a 5D (which I returned).

It's a pity Canon doesn't upgrade all new lenses with the new 3rd Gen IS. It is a marvel.
I have the 24-70L which I'll keep but will be used perhaps 80% less now as this lens has replaced it on 1.25 crop 1D2 for non vignetting all purpose walkaround lens. You cannot downplay 280g less weight on a long trek, while it's wrapped round your neck or shoulders.
Testimony to the DWR sealing. I was caught in a thunderstorm while trekking, the rain so heavy visibility was down to maybe 15m at one point. My buddies with Tamron and Sigma had malfunctioning lenses due to leaks, while mine worked flawlessly in adverse weather conditions. That alone is worth $700.
AF speed is as fast as the 24-70L and the pictures are very comparable except for the distortion from 24-50. It's slight and acceptable but still is an annoyance. I think people shooting architecture may object to it but landscape and general photography wouldn't object to it as much.
Mine was under recall so I had Canon calibrate the repaired to my 1D2. It didn't affect the lens at all since it was a swap except for the mount. It's not a perfect lens but excellent build and good images offset the slight distortion that is prevalent in the wider lengths. It also vignettes pretty badly on a 5D, but I returned the camera and kept the lens. It is the best outdoor zoom lens available for my 1D2, for my needs.

A little birefringence throughout the aperture range, 185-200 a little less sharp wide open

Had one for 1.5 years then it was stolen. The new replacement performs as well except wide open from 185-200. The birefringence is also new but that's with the 1DII, which apparently has problems with a few lenses and birefringence.